1. Field of Inventions
The present inventions relate generally to devices for performing therapeutic operations on body tissue.
2. Description of the Related Art
There are many instances where therapeutic elements must be positioned adjacent to body tissue. One instance involves the formation of therapeutic lesions to the treat cardiac conditions such as atrial fibrillation, atrial flutter and arrhythmia. Therapeutic lesions, which may also be used to treat conditions in other regions of the body such as the prostate, liver, brain, gall bladder, uterus and other solid organs, are typically formed by ablating tissue.
Cryogenic cooling devices are one example of the devices that have been used to form lesions in tissue. During the cryo-ablation of soft tissue (i.e. tissue other than blood, bone and connective tissue), ice crystals disrupt cell and organelle membranes and it is the disruption that kills the tissue. A cryogenic element, such as a balloon or hollow metal tip, is carried on the distal end of a catheter or surgical probe (referred to herein collectively as “probes”), placed in contact with tissue and cooled to a temperature that will cause tissue death. The cryogenic element may be cooled by a variety of techniques. One technique employs the Joule-Thompson (“JT”) effect. Here, cryogenic cooling occurs as a result of a rapid decrease of gas pressure that occurs within the therapeutic element. Pressurized cryogenic fluid, such as liquid nitrous oxide, is directed into the therapeutic element where it undergoes rapid phase change and a rapid expansion of the gas from a high-pressure to a lower pressure state. The reaction is endothermic and produces temperatures as low as minus 70° C. at the therapeutic element. In some instances, the cryogenic fluid is pre-cooled in order to increase the cooling power delivered to the targeted tissue. The cryogenic element may also be cooled by directing super-cooled fluid through the catheter or surgical probe to the cryogenic element. Here, the temperature at the therapeutic element can be as low as minus 100° C. when it enters the patient.
The present inventor has determined that conventional cryogenic cooling devices are susceptible to improvement. For example, the present inventor has determined that conventional cryogenic cooling devices can damage tissue at the insertion area, i.e. the area at which the probe is inserted into the patient. In the context of cryogenic probes, which utilize the JT effect, the tissue in the insertion area may be damaged by the nitrous oxide as it returns to the proximal end of probe in its extremely cold gaseous state. Super-cooled fluid, on the other hand, is more likely to damage tissue in the insertion area as it enters the patient. The insertion area, and the tissue contained therein, will vary from procedure to procedure. In the context of cardiovascular therapies which require a catheter to be passed through the femoral vein to a chamber within the heart, for example, the insertion area would include the tissue between the outer surface of the patient's thigh and the femoral vein. With respect to surgical probes, conventional cryogenic cooling devices can damage tissue at the insertion area, i.e. the area at which the probe is inserted into the patient, especially in minimally invasive procedures, in a manner similar to that observed with percutaneous catheter procedures. The proximal or mid-portion of the surgical cryogenic probes can also freeze soft tissues not intended to be ablated. For example, lung tissue can be unintentionally damaged when heart tissue is targeted. When liver tumors are targeted for cryo-ablation, the abdominal wall, spleen, pancreas or stomach could be injured.